Design of Tailings Dams

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Design of Tailings Dams

Design of Tailings Dams

We are looking at the design of a Tailings Dam - also called a slimes dam, residue dam, or mine dump. It is proposed as 500 m diameter x 50 m high.

I have been told that Tailings Dams collapse at heights in excess of 30 m. The difference betwen an earth-fill dam, and a tailings dam is that earth-fill dams are designed to retain full depth of water, while tailings dams are huge settling ponds which are designed to silt up.

RE: Design of Tailings Dams

When I just finished college(15 years ago), I was sent up to a mine to watch over the constuction of 3 tailing dams. It was holding the tailings from a platimum and paladium mine. Of course being in the middle of nowhere, we tried to use site materials. The first was a earthen dam. It was only about 15 feet high with a standard dam slopes. I think the hydraulic conductiviy was about 10-7cm/sec.The second was an earthen dam with a concrete keyway because it was on top of bedrock. Similiar design but higher. The third one was the biggest (50'high) and it used the waste rock on the one side and then used the tailings as a cusion on the rock face. On top of the tailing was a HDPE liner. Another layer of tailings and then a geo mat.The tailings were all a consistent size so they can hold a lot of water but it would drain fairly easily. I remember the backhoe was digging tailings and then walked out and shut down for the night. It was below freezing that night and the tailing in the track froze and there was no way of cleaning it. They had 50 tiger torches going trying to thaw it out.I would think with the extra weight of the tailing, you would need gentler slope on the upstream side but other than that would be a standard dam.Good luck on your search, its interesting stuff.

RE: Design of Tailings Dams

The starter walls are usually well designed, and supervised with care during construction. After that it's up to the Mining House to build each lift according to some basic guidelines. The assumption is that the slurry coming into the dam will separate out into settled particles and supernatant water. So the wet head should not be more than 1 or 2 m.

This concept may work for a number of lifts to say 20 - 25 m. But after several years the construction technique may be relaxed a little. And, in some cases this leads to breaching of the dam wall and its consequences.

Earthfill dams don't like stormwater so this is a serious consideration in valley fill dams, but less threatening with contained above ground basin style dams.

With higher lifts of 30 - 50 m the technology is available in water dams, but it would appear has not yet been adapted to tailings dams. The key being the deposition aspect, and the lower levels of wet hydraulic face.

Are there aspects of chimney drains, and other standard dam technology which we should be applying?

Do we adequately understand piezometric effects, and flow nets at high dam wall situations?

RE: Design of Tailings Dams

Tailings dams have gone and will continue to be constructed to great heights - 50 m has been exceeded. There are several methods of constructing - upstream and downstream being the most simplistic. It is an art to a large degree and one should never attempt to design one without prior experience as well as the environmental concerns now being as great as they are. There have been a few collasal failures. There is a book published by Bi-Tech by Maciej Syzmanski abou the design of tailings dams (you will find them in the Geotechnical news of the ISSMFE - or whatever it is initialed these days.). Also a chap by the name of Vicks has done extensive published work. There are a myriad of great designers - many in Canada - but I suggest, as it has been pointed out before - hire a group that has experience, for your own initial information, buy Syzmanski's book and do some on-line research.

RE: Design of Tailings Dams

I think a lot of the limit to the height of tailings dams is economics. Whereas tall earthen embankments may have hydroelectric, water supply, flood protection and other reasons for governement funding, tailings dams typically have to be on land controlled by the mine. A lot of tailings dams are also in flat areas where you don't need and couldn't build huge embankments.

My question would be what is the mine, what type of tailings, environmental protections needed, and how do you intend to reclaim the tailings dam when it reaches design life.

As for for collapse, I have seen limits for upstream and centerline construction but it depends on your tailings and construction material. Upstream construction is tempting for high dams, just because of the volume of fill needed, but the uncertainties for your factor of safety get a lot higher.

RE: Design of Tailings Dams

Engineering is more than just technical calculations. Sometimes it takes strategy and tactics to convince upline management (who have had 30 years experience designing tailings dams) that there might be more to it than detailing 2 m starter walls. You cannot outsource until you know more than management - and have the hard facts to back up any challenge.

I'm suprised that this would be a geotechnical topic and not a civil engineering one. I've just had some interesting exchanges with geotechnical engineers who understand rocks, but have only a vague interest in soil technology.

Dams holding water are designed to resist water pressure, percolation, and seepage problems. Dams trapping mine sediment do not require such protective measures to the same extent (and the dams are usually unlined). As you build higher over a number of years control over construction of each lift can become less stringent, with counter drainage inserts less effective. Sometimes the decant system collapses and you start capturing process water and sulphuric acid with nowhere to go.

RE: Design of Tailings Dams

Since most tailings dams have a small watershed and relatively small if any inflow from storm runoff, hydrology and civil engineering is not a large issue with these dams. For typical flood control, hydroelectric or water reservoir dams, hydrology and hydraulic engineering will become a much larger part of the engineering design process required. From my experience (as a civil), these dams are nearly always completely in the realm of the geotechnical engineers that I work with. But being owned privately, many owners may require that the mine engineer design it and likely not allow much interference from a geotechnical consultant - until one of these dams fails and then suddenly, the mine owner will call in a geotechnical expert to repair the dam...

I believe that water pressure, percolation and seepage problems are generally the factors causing failure of tailings dams, and as you indicate - partially because safety factors in design are not particularly high and quality control during frequent raisings of the dam is often an afterthought. In addition, since the upstream side of the dam is difficult to inspect after a number of years of infill, inspection is largely limited to the downstream side. It also appears that since most of these dams are privately owned, that regulation and regular inspections are also more difficult to achieve.

RE: Design of Tailings Dams

I actually thought that the standard now was lined ponds due to EPA in the US. Maybe not in coal country but then you have the whole issue of weight against coal seams. I agree that tailings dams are almost wholly in the realm of geotechnical engineers. I thought civils were there for the 5% H and H work, the client interaction, and the 10% overhead (and I am a civil), leaving design and construction inspection for the geotechs.

RE: Design of Tailings Dams

My first gig was working for a group specializing in tailings dams based in Vancouver, BC, Canada. Typically, we looked at (for brand-new projects) all stages of the dam construction through closure and abandonment. Environmental standards were typically in line with North America.

A few really large jobs stand out in my memory, on the order of 200-300 Mm3 just for the dam, and up to 700 (if I am recalling correctly) Mm3 overall.

That having been said, our group was kept up on the analysis behind large dam failures (Omai, Marcopper) that occurred during my time there. IIRC, the Omai failure was due to inadequate foundation preparation for the dam. I've got the article in my files somewhere...

While I don't know of any tailings-specific design guidance, the International Commision on Large Dams (ICOLD) and National Center for Earthquake Engineering Research (NCEER) publications and research were used as reference material.

Hope this helps.

Jeff

RE: Design of Tailings Dams

Thanks jdonville,1 - the core issue on this thread is addressed by the US Environmental Protection Agency, Office of Solid Waste Washington - Technical Report on the Design and Evaluation of Tailings Dams. I have the download, but lost the URL.

RE: Design of Tailings Dams

I agree with most, if not all, of what has been said. Primary design professional would be a geotechnical engineer, at least in the US.

Practical maximum height of the structure will depend on many many items some of which are listed next:1. type of mine tailings (coal waste is not likely to support as high a dam as hard rock waste).2. topography3. foundation conditions4. earthquake ground motions5. downstream developement6. enviormental considerations and regulations

I'm sure there are many other ones that I missed.

Most times, at least in the US, tailings dams are sized to meet the anticipated needs of the mine for the next two to ten years (maybe longer if you have a really forward looking mine operator). The dam is used to sort the tailings and when it is about to get full, they start looking at how to expand the existing dam. As has been pointed out this can be done in many ways, however, the most common is either upstream or downstream construction. Both have advanatages and disadvantages.

In the end design of tailings dams is very much like other dams, however, you generally have less control over the materials that will be placed within and as has been pointed out, quality control is sometimes not as tight as it could be.

Now back to your original question. It appears that you are looking at building a ring dam with a height of 50m. This would mean that H&H would have little to do with the dam. Therefore, a geotechnical (dam) engineer is what you would need. They should have worked with similar tailings in the past and preferably in the same geographic area as the proposed dam.

I would expect the biggest concerns to be (not in any specific order):1. foundation preparation beneath the 50m dam2. the ability of the dam material to relieve the pore pressure from the filling operation3. strength of both the dam and tailings materials4. earthquake induced ground motion5. rate of proposed filling

Future expansion of this type of dam would likely be by the downstream method, so siting of the structure would be key.